My experience with nail polish is that it leaves residue around the firing pin hole and on the bolt face. Whether or not this is a problem waiting to happen...I am not sure. There are several commercial sealants available for primer pockets. One is named Roy's Primer Sealant. I have used that on primers, but not tried any case mouths. Midway carries another as well, but the name escapes me.

Mil-spec ammo sometimes uses asphaltic varnish on the case neck. I don't know how you would duplicate that easily.

Jeff

edit to add that Markron Primer Sealant is carried by Midway. The ad appears to indicate that you can seal the bullet too, but I don't know how you do that easily.